1. Field of the Invention
The present invention relates to a monitor lens for a portable telephone, a monitor lens for a PDA or PC, and an imaging lens used in a television camera, or the like, using CCD and CMOS devices.
2. Description of Related Art
It has been proposed to use plastic lenses, and lenses permitting cost reduction and compactification, for imaging lenses for CCDs of this kind.
However, although it is certainly possible to achieve reduction in the number of lenses and size reductions, there have been observed lenses wherein, even if a shorter optical path is successfully achieved, the distortion aberration is increased, and the subject becomes distorted, or lenses wherein the outer diameter of the lens is increased, or the overall length is increased, in order to correct distortion aberration, or other aberrations, and hence compactification cannot be achieved.
In the future, cameras with imaging systems using CCD or CMOS devices, will become extremely small, and hence there will be demand for extremely short optical paths in the associated optical systems.
The prior art cannot be seen as providing compactification and satisfactorily contributing to aberration compensation in order to obtain suitable aberration corresponding to the compactification.
From here on, cameras using CCD and CMOS devices will become increasingly compactified, and optical lens systems used in miniature equipment, such as portable telephones, PDA equipment, and the like, will have shorter optical paths than lenses of this kind in the prior art, and furthermore, such lenses will not be usable unless distortion aberration is also corrected.
It is an object of the present invention to provide an extremely small imaging lens having a short optical path, low distortion aberration and high performance, which can be used in cameras, televisions, and the like, using CCD and CMOS devices, and comprises a small number of lenses, namely, a two-lens composition, whilst maintaining a luminosity of approximately f2.8.
It is a further object of the present invention to provide an imaging lens which achieves weight reduction by constituting all of the lens by means of plastic material.
In order to achieve the aforementioned objects, in the present invention, the lens system is constituted as described below (see FIG. 1).
Namely, the imaging lens according to the present invention comprises, in sequence from the object side to the image side: an aperture diaphragm, a meniscus-shaped first lens having positive power with a concave face oriented towards the object side, a diaphragm, and a meniscus-shaped second lens having a concave face oriented towards the image side, and hence the lens comprises a total of two lenses, namely, the first lens and the second lens.
In the present invention, by adopting a lens system wherein at least one face of the first lens is an aspherical face, and one or more faces of the second lens is an aspherical face, in such a manner that the imaging lens as a whole is a lens system comprising at least two aspherical faces, good aberration values are obtained, and furthermore, effective use of plastic can be used when employing plastic lenses.
In the present invention, in addition to the shape characteristics, the following compositional conditions, (1), (2) and (3) must be satisfied.
0.01 less than |f1|/|f2| less than 0.6xe2x80x83xe2x80x83(1)
0.3f less than |R2| less than 0.6fxe2x80x83xe2x80x83(2)
0.5f less than D0 less than 1.5fxe2x80x83xe2x80x83(3)
where
f: focal length of whole lens
f1: focal length of first lens
f2: focal length of second lens
D0: distance from the aperture diaphragm face to the second face of second lens (total central length of lens)
R2: radius of curvature of image side of first lens
In the present invention, the first and second lenses may both be made from glass or from resin.
Moreover, in the present invention, both faces of the first lens may be aspherical faces, and the second face of the second lens may be an aspherical face.
According to the present invention, a lens for a CCD or CMOS device is constituted by means of a small number of lenses, namely, a two-lens composition, and in order to achieve suitable brightness in a compact structure, aspherical faces are employed in at least two of the lens faces.
Condition (1) determines the power distribution of the first lens and second lens, and if the value of |f1|/|f2| becomes less than the minimum limit in condition, then the power of the first lens will strengthen and the power of the second lens will weaken, and consequently, it will become difficult to correct spherical face aberration, coma aberration, and distortion aberration generated by the first lens.
Moreover, if the value of |f1|/|f2| exceeds the maximum limit of condition (1), then the power of the first lens weakens, and in order to shorten the focal length (f) and back focus (bf), it is necessary to increase the power of the second lens, thereby making it more difficult to correct distortion aberration and coma aberration generated by the second lens, and hence a good image cannot be obtained.
Condition (2) ensures that a suitable optical length is obtained, whilst maintaining good aberration.
If the minimum limit is exceeded, then the radius of curvature of image side of the first lens will form a strongly convex face, meaning that spherical face aberration, coma aberration, and distortion aberration will increase, and it will be difficult to correct this aberration, even if an aspherical surface is adopted.
Moreover, in a lens having a short focal length (f), the radius of curvature will become too small and will become impossible to process.
If the upper limit is exceeded, then the radius of curvature of image side of the first lens forms a weak convex surface, and the power of the first lens will weaken, thereby making it difficult to adjust the correction of aberration in the first lens, the focal length (f), and the length of the optical path.
Even if R2 is formed as an aspherical face, it is difficult to correct aberration in this face, and ultimately, good images cannot be obtained.
Condition (3) stipulates the size of the lens system, whilst simultaneously maintaining the peripheral amount of light, and if the total length of the lens (D0) is less than the lower limit of condition (3), then in the case of a meniscus-shaped lens, the Petzval sum will increase, which is undesirable. Moreover, if the focal length is adjusted by reducing the radius of curvature of the second face (R2) of the first lens in order to lengthen the focal length of the whole lens system, then the spherical face aberration, coma aberration and distortion aberration will increase, and it will not be possible to obtain a good image.
If the total length of the lens (D0) exceeds the maximum limit of the condition (3), then the lens as a whole will be long and the ratio of peripheral light will decline. In order to correct this, the external diameter of the second lens must be increased, but the increased overall length and external dimensions of the lens make it difficult to achieve compactification.
By means of the three conditions (1) to (3) above, a compact imaging lens having excellent properties is obtained.
Moreover, the first lens may be formed by a double-convex lens having positive power wherein both convex faces are oriented towards the object side. In this case, no diaphragm is provided between the first lens and the second lens. The remaining composition and conditions according to the present invention may be adopted in this case.